The invention relates to plastic containers for liquids or gases.
Composite materials, that is, laminated bodies made of various plastics, are already known, and in many fields they have replaced the materials previously typically used. However, it has not yet been possible to use such composite plastics or sandwich materials for all fields of application, because either their mechanical properties or other characteristics did not allow their use in place of metal or ceramic materials. Among these fields of application are for instance the manufacture of water tanks, in particular hot-water heaters or tanks for other aqueous liquids, which to a large extent are still today made of enameled steel. Enameled steel tanks are markedly vulnerable to corrosion, however, because hairline cracks form in the enamel film, and the equipment undergoes considerable corrosion relatively quickly at temperatures of approximately 601/2.degree. C. or below. Experience has shown that such water tanks of steel last for only about 2 to 3 years and then must be replaced because of corrosion damage. It is already known from German Patent Application No. DE-OS 33 42 386, however, that instead of steel tanks, tanks made of plastic can be used; these tanks comprise an inner layer of modified polyphenylene oxide, i.e., PPO, and an outer layer of glass-fiber-reinforced polyester resin. The glass-fiber-reinforced polyester resins have a glass fiber content of approximately 25 to 30%, and in special cases approximately 60 to 80%. Practical tests have shown that such storage containers have a considerably longer service life than metal storage tanks and sometimes can be used for up to 10 years without corrosion. This is all the more unexpected since it is known per se that polyphenylene oxide (PPO) has only a very low water absorption capacity, yet at higher temperatures an oxidative destruction is nevertheless ascertainable. Despite the elevated temperature typical in the tanks, however, it has been found that PPO in the modified form has excellent resistance to oxidation and hydrolysis when used as an internal liner.
In long-term tests of the above-described plastic containers used as tanks, however, it has been found that unexpectedly, embrittlement of the PPO inner liner was ascertainable after relatively long use, which if the container is stressed by jarring or vibration, as can for instance happen even from traffic on busy streets, can lead to hairline cracks and hence to leaking of the container. Only with extensive tests was it possible to ascertain that the cause is the styrene content that the customary polyester reaction resins available in commerce have as a rule. The unpolymerized styrene in the outer laminate layer attacks the PPO liner and hence causes an impairment of its mechanical properties.
It is therefore the object of the invention to develop plastic containers of the above-described type that do not have the demonstrated disadvantages and which are suitable as tanks, particularly as hot water heaters and also as unpressurized containers, particularly the so-called expansion tanks for hot water heating systems.